According to an American Association for Cancer Research-led study, bacteria may promote pancreatic cancer by suppressing the immune system. The study was conducted by George Miller and Deepak Saxena.

"The gut microbiome has been studied in many different cancers, including liver and colorectal cancer, and is shown to affect cancer progression," said Miller. "Because the pancreas is remote from the gut, it is considered a sterile organ, and there haven't been many studies that looked at the role of the gut microbiome in pancreatic cancer."

The researchers also compared fecal samples from 32 patients with pancreatic ductal adenocarcinoma with fecal samples from 31 normal individuals and found that the bacterial composition of cancer patients was distinct from that of normal individuals.

"The bacterial composition was more diverse in the fecal samples than from cancer patients," noted Miller.

"The dysbiosis [imbalance] in the gut microbiome can potentially be used as a biomarker to define a high-risk population," noted Saxena. Among the more abundant strains of bacteria found in pancreatic cancer patients were Proteobacteria, Bacteroidetes, and Firmicutes.

In mouse studies, the team demonstrated that bacteria translocate from the gut to the pancreas during pancreatic cancer. With further studies, the researchers showed that eliminating these bacteria using antimicrobial treatment slowed the progression of pancreatic cancer and lowered the tumor burden by about 50 percent.

This process also affected T-cell differentiation, leading to increased T-cell infiltration into the tumor and reduction in myeloid-derived suppressor cell (MDSC) population. Antimicrobial treatment also resulted in increased expression of PD1 on CD4+ and CD8+ T cells within the tumors.

Reintroduction of bacteria in antimicrobial treated mice reversed the tumor protection and reduced the immunogenicity of the tumors, suggesting that the microbiome promotes pancreatic ductal adenocarcinoma by inducing immune suppression in the tumor.

"We were surprised to see that the human pancreatic tissue samples had an active microbiome," said Saxena. "And we found that not only are there bacteria in the pancreas but the bacterial load is significantly higher in pancreatic cancer tissue compared to normal pancreas tissue."

"Our studies show that the bacteria may serve both as biomarkers of increased risk for pancreatic cancer as well as potential therapeutic targets," said Miller. "We believe that targeting the microbiome in patients with pancreatic cancer can make immunotherapy effective."

The researchers also found that combining antimicrobial treatment with an anti-PD1 immunotherapy resulted in enhanced CD4+ and CD8+ T-cell activation in mice, suggesting that such a combination is a potential treatment option for pancreatic ductal adenocarcinoma.

Saxena noted, "Pancreatic cancer is a very aggressive disease with a five-year survival rate of a dismal 8.2 percent. Extending the life of these patients by manipulating the microbiome and decelerating tumor progression would be a significant step forward in managing this deadly disease."

The findings from this study are published in the Cancer Discovery, a journal of the American Association for Cancer Research